Joint range-modulator and spot optimization for Bragg-peak proton FLASH radiotherapy

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Biology

Background:

• Ultra-high-dose-rate (UHDR) radiation therapy, particularly proton FLASH (pFLASH), shows potential for reduced organ-at-risk (OAR) toxicity.
• Current pFLASH treatment planning involves a two-step process: intensity-modulated proton therapy (IMPT) plan generation followed by patient-specific range modulator (PSRM) optimization.
• The interplay between spot weights and PSRM design in pFLASH therapy is not fully understood, potentially limiting dosimetric and radiobiological benefits.

Purpose of the Study:

• To introduce a novel alternating optimization framework, Joint Range-Modulator and Spot Optimization (JRSO), for simultaneous optimization of PSRM and spot weights.
• To enhance the plan quality of conformal pFLASH radiotherapy through integrated optimization.

Main Methods:

• JRSO framework developed for simultaneous optimization, removing the need for a one-to-one correspondence between beam spots and PSRM pins.
• Proton beam diffusion modeled using a Gaussian function.
• Iterative alternating updates of PSRM design and spot weights, starting from an IMPT-derived solution, while adhering to constraints like minimum monitor units (MMU).

Main Results:

• JRSO demonstrated superior plan quality compared to conventional methods.
• In a head-and-neck (HN) case, JRSO reduced the objective function value (0.46 to 0.26), lowered maximum target dose (117.6% to 107.1%), improved conformity index (0.74 to 0.87), and decreased ROI effective dose (6.50 Gy to 6.10 Gy).

Conclusions:

• The proposed JRSO method offers an advancement for conformal pFLASH radiotherapy planning.
• JRSO outperforms the conventional approach and may broaden PSRM applicability in complex clinical scenarios, including spot-pin misalignments.
• Numerical results confirm the robustness and efficiency of the JRSO method.